Week-7 Challenge: DC Motor Control

Q1) Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Ans:- BJT H-bridge :-

An H-bridge is a simple circuit that we control a DC motor to go backward or forward depending on how we connect the plus and the minus.

If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction.

If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction.

A side-effect of how a motor works is that the motor will also generate electrical energy. When you disable the transistors to stop running the motor, this energy needs to be released in some way.

If you add diodes in the reverse direction for the transistors, you give a path for the current to take to release this energy. Without them, you risk that the voltage rises and damages your transistors.

Pulse generator :-

The Pulse Generator block generates square wave pulses at regular intervals. The block waveform parameters, Amplitude, Pulse Width, Period, and Phase delay, determine the shape of the output waveform.

The Pulse generator given by a pulse width of 75% and period of 2e-3 seconds. The motor parameters of the torque and field voltage and armature voltages are taken as default values of the model.

(b) From the above figure of armature current is shoot – up suddenly increase in magnitude and after that suddenly decrease in magnitude up to 75% to 50% and the output of the current is shown in figure and in second figure pulse with modulation shows the no suddenly change the in magnitude of armature current and if we further reduce magnitude then negative of the rise will get reduce and it is clear the from the above figure if we reduce the duty cycle ratio then we obtain the smooth variation in the output of the graph and in the above graph pulse width modulation is 40%.

Q1) (C)

Chopper :- A chopper is a static device that converts fixed DC input voltage to a variable DC output voltage. It is basically a high speed ON/OFF semiconductor switch. It may be thought of as DC equivalent of an AC transformer since they behave in an identical manner. Chopper is fed through a constant DC voltage source and its output is variable DC voltage. The average value of output DC voltage may be less than or higher than the input DC voltage source. This switch can be made ON and OFF at a very high speed. In this way, the load may be connected and disconnected from the supply source Vs. When the switch is ON, the load voltage is equal to the source voltage Vs and when the switch is OFF, load voltage becomes equal to ZERO. Thus, a chopped voltage across the load is obtained.

Quadrant Operation of Chopper

a chopper is a semiconductor power electronic device. Again, a semiconductor power device used in chopper circuit are unilateral devices. This means, the polarities of output voltage and direction of output current is restricted. If we want o change the polarity of either output voltage or current or both, we need to change the circuit configuration of chopper.

he current i_o and V_o are positive. If the direction of the i_o or v_o changes from the shown direction, it will be assumed negative. The circuit diagram shown above correspond to step-down chopper. In step-down chopper the load voltage and load current is always in the direction shown in the figure. This means, i_o and V_o are always positive and hence the operation of chopper is in First Quadrant.

Similarly, in second quadrant operation, the V_o will remain positive but the load current i_o will be negative. A step-up chopper operates in second quadrant during OFF period. It may be noted that the circuit configuration for step-up and step-down choppers are different. In fact, we need to change the circuit to suit our requirement for a particular quadrant operation. However, there exists a circuit configuration which can be used to operate a chopper in any of the four quadrants. This configuration is called Class-E Chopper.

In third quadrant, both i_o and V_o will be negative and in fourth quadrant, the I_o will remain positive but the V_o will be negative.

Thus, quadrant operation of chopper is nothing but to change the polarity of output voltage and current by means of suitable circuit configuration.

Compression between chopper and H – Bridge model

Four Quadrant chopper

:- The motor can be operated in all the four different modes (forward motoring, reverse motoring, forward braking and reverse braking)

:- In this the vehicle control is better Regenerative braking can be achieved.

:- The signals given to the four quadrant operation are discrete.

:- The four-quadrant chopper widely used in the automobile application for the control of DC motor speed and direction.

H-bridge model

:- In this model we have only two modes forward and reverse by changing the direction of motor.

:- In this the vehicle control are less

:- In this regenerative braking is not possible.

:- Here the signals are continuous.

:- The H-bridge is used in the robotics and electro-mechanical devices to control the speed and direction of the DC motor at low level speed.

2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor)

Ans:- The Two-Quadrant Chopper DC Drive (DC6) block represents a two-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with two-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The main advantages of this drive, compared with other DC drives, are its implementation simplicity and that it can operate in two quadrants (forward motoring and reverse regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, for all two-quadrant DC drives, reversible and regenerative operations (reverse motoring and forward regeneration), which are required in most DC drives, cannot be obtained.

The two-quadrant chopper operates in the first and second quadrants, allowing positive and negative output current. The Control subsystem implements a simple PI-based control algorithm for controlling the output current.

3. Explain in a brief about operation of BLDC motor.

Ans :-

BLDC Motor operation is based on the attraction or repulsion between magnetic poles. Using the three-phase motor the process starts when current flows through one of the three stator windings and generates a magnetic pole that attracts the closest permanent magnet of opposite pole.

The rotor will move if the current shifts to an adjacent winding. Sequentially charging each winding will cause the rotor to follow in a rotating field. The torque in this example depends on the current amplitude and the number of turns on the stator windings, the strength and the size of the permanent magnets, the air gap between the rotor and the windings, and the length of the rotating arm

Advantage of BLDC Motor:

Compared with a brushed DC motor or an induction motor, the BLDC motor has many advantages:

• Higher efficiency and reliability
• Lower acoustic noise
• Smaller and lighter
• Greater dynamic response
• Better speed versus torque characteristics
• Higher speed range
• Longer life